1. Field of the Invention
The present invention relates to a light guide module plate, and more particularly to a light guide module having a light diffusion arrangement over a light emitting surface, thereby providing uniform luminance distribution over the entire light emitting surface.
2. Description of Related Art
A liquid crystal display is capable of displaying a clear and sharp image over a wide area. It is thus used with various devices in which a message or picture needs to be illustrated. However, a liquid crystal itself does not emit light, therefore, it has to be back-lit by a light source to display the messages and/or pictures shown there.
In an ideal liquid crystal display, the backlight provides light evenly across the entire surface. In addition, the apparatus has to meet the requirements of being small in size, light in weight, and bright enough, while having a low power consumption.
U.S. Pat. No. 5,438,484 issued to Kanda et al. discloses a surface lighting device. A variety of prior art surface lighting devices are disclosed in FIGS. 1 to 5 of the Kanda patent. The light source arrangements in the surface lighting devices of FIGS. 1 to 5 are generally referred to as “edge-type” light sources. Kanda describes the disadvantages of the prior art surface lighting devices in detail, i.e., that the surface areas closer to the light sources are brighter than the central areas. According to Kanda's explanation in the specification, “However, as described above, the surface lighting device of an edge-type has a low luminance in the central portion between the light sources and a high luminance in the vicinity of the light sources as indicated by a broken line C shown in FIG. 9. This is because the light sources 1a and 1b emit diffusion light and make the vicinity of the light sources 1a and 1b bright while the light emitted from the light sources 1a and 1b mostly reach the opposite light source 1b and 1a to be diffused, respectively, thus making the vicinity of the light sources 1a and 1b brighter. As a result, it is inevitable that the effective light range (effective emission surface) of the foregoing lighting device will become narrower because its overall luminance must be adjusted to latch evenly as a backlight with the lowered luminance between the central portion of the light sources 1a and 1b. Thus, a problem is encountered that the light utilization efficiency for the apparatus as a whole is reduced.” See Column 2, lines 31 to 49.
Kanda provides a solution, as shown in FIGS. 11 to 16, by providing “a light guide configured by a plural light transmitting members joined together, so that the junction surface therebetween crosses the light emitting surface.” As a result, according to Kanda, the luminance emitted from edge-type light sources is evenly distributed across the entire area.
Kanda provides another solution in FIGS. 17 to 23, typically shown in FIG. 23. In this application, the light source is arranged directly behind the diffusion board, instead of at the edge of a light guide, as shown in FIG. 1 of the Kanda patent. However, although this arrangement does provides a brighter central displaying area, it creates a problem of color chromaticity across the liquid crystal display. As explained by Kanda in Column 12, lines 19–49. Kanda then uses a “light source having preferably be more blueish than the standard color” to solve the “yellowish” problem.
Aside from use of the “blueish light source”, it is noted that a “light curtain”, reference numeral 14 of FIG. 22, has also been used to reduce the luminance projected toward the display area immediately in front of the light source. It should be easy to appreciate that the more parts used within the liquid crystal display, the more laborious the effort needed to assemble the display. No doubt, the size and weight of the liquid crystal display will inevitably be increased.
U.S. Pat. No. 5,881,201 issued to Khanarian discloses improved light pipes for backlighting liquid crystal display devices. The light pipes comprise transparent polymers with scattering centers. A preferred composition for such light pipes comprises a cycloolefin polymer containing scattering centers from suitable elastomers and inorganic fillers. The inventive light pipes offer superior scattering efficiency as well as spatial uniformity of scattering and uniformity of scattering across a wide wavelength range.
Referring to FIG. 7, Taiwan Utility Patent Publication No. 368081 discloses a backlight module 100 which generally includes a transparent light guide 110 with a reflective light enhancer 120 attached to a bottom surface thereof. The backlight module 100 further includes a diffusion film 130 arranged on a top surface of the light guide 110. A fluorescent light 140 is arranged at an end of the light guide 110. It can be readily appreciated that the light guide 110 is provided with a plurality of elongate printed lines 111 diverging away from the light source 140 located at the end of the light guide 110. The backlight module 100 further includes an end reflective enhancer 150 to homogeneously reflect the light beams so as to provide a uniform distribution of luminance over the light guide 110.
The diffusion film 130 functions to homogenously diffuse the light beams projected from the light source 140. However, before the light beams reach the diffusion film 130, the light beams have to travel through the light guide 110, and be reflected by the reflective light enhancer 120. It should be noted that before the light beams reach the diffusion film 130, the light beams have to first be emitted from the light guide 110, and then must enter the diffusion film 130. The light guide 110 and the diffusion film 130 have different refractive indices and surely the energy of the light beam is exhausted during the transition. In addition, the diffusion film 130 is a separate element in addition to the light guide 110, has to be manufactured separately. As such, the overall cost is inevitably increased.